Microphthalmia-Anophthalmia-Coloboma (MAC) disorders are a heterogeneous spectrum of congenital ocular defects that result in severe structural malformations of the eye. MAC phenotypes can be isolated, or they can be components of a variety of syndromic disorders. Reported incidences vary widely across the globe but approximate ranges are 2.13 per 10,000 live births for anophthalmia and microphthalmia, to 2.6 to 7.5 per 10,000 for colobomata. Colobomata are also a component of over 50 human genetic disorders, where they are often associated with microphthalmia. Despite a significant amount of genetic research to identify MAC loci, causative mutations have been identified in very few cases, and for those that have, little is known about how the affected proteins facilitate normal eye development. Recently, several different mutations were identified in the MAB21L2 gene in human MAC patients [Rainger et al., 2014]. MAB21L2 has not been previously linked to MAC spectrum disorders or to any human congenital disorder. MAB21L2 encodes a protein of unknown biochemical function and it is unknown how the mutations affect MAB21L2 activity. Previous work in our laboratory identified a zebrafish mab21l2 mutant, and ocular defects in the mutant resemble those in human MAC patients. Research in this proposal utilizes zebrafish mab21l2 mutants as a translational model system through which we can identify the molecular underpinnings of human MAB21L2 mutations and determine the cellular function of MAB21L2 during normal eye development. We test the hypothesis that MAB21L2 activity is required for normal proliferation within the optic cup and that Mab21l2 deficiencies result in microphthalmia which prevents apposition of the lateral edges of the choroid fissure, thereby contributing to colobomata. We combine RNA-Seq, chromatin-association assays and LC-MS/MS proteomics to determine the in vivo function of MAB21L2 and elucidate the MAB21L2-dependent gene regulatory network and protein interactome underlying normal eye formation. These experiments fit the mission of the NIH and the NEI because they have direct relevance to furthering our understanding of early eye development and MAC spectrum disorders. Furthermore, they develop a zebrafish mutant model as an in vivo translational platform for determining the pathogenicity of human mutations. Given the prospects of personalized medicine, and the ease/low-cost of exome and whole-genome sequencing, mutations in genes encoding proteins for which little functional information exists will be increasingly identified. The pipeline established in this proposal will serve as a useful in vivo paradigm for elucidating the molecular underpinnings of these human mutations, and for rapidly determining the functions of the proteins they encode.